This is a question that you must answer yourself … Both the rule books of the fifth and sixth edition establish that "there is a way to calculate the income", but they are not rules, only suggestions.
This is an unfortunate example of "do whatever you want about the rules", but you may notice that the "assets = annual income" mode of the companion is consistent with the book of basic rules of the fifth edition.
I also think that 5 times the annual income seems too much. It could be realistic for high class characters, not really for the rest.
If you are going to run MoN, you could use the trick that was introduced in the rewrite of the campaign:
Jackson got a large sum of money (which is explained in the prequel scenarios) and has a trust fund that gives investigators in his will if they "continue with their work," which makes the financial part of the researchers' problem be a kind of debatable.
You could elaborate this idea to incorporate it into your environment (I do not think managing physical resources should be a big problem in a CoC campaign, but, of course, you can do as you see fit).